Two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED/EDM1) result from massive intracellular retention of mutant COMP (MT-COMP) in the endoplasmic reticulum (ER) which causes premature chondrocyte death during bone development. The focus of this research is to determine the cellular mechanism by which MT- COMP promotes the chondrocyte cellular pathology and to determine whether various pharmacologic agents inhibit or reverse the pathology. In previous studies, we have shown that: 1) mutations in COMP cause skeletal dysplasias, 2) mutant COMP interferes with the export of COMP as well as type IX collagen and matrilin-3 proteins to the extracellular matrix, 3) intracellular matrix is inappropriately assembled in giant rER cisternae, 4) the PSACH cartilage matrix is deficient in ECM proteins and 5) the intracellular retention of COMP causes premature chondrocyte cell death. While our in vitro chondrocyte model system indicates that mutant COMP is not degraded by the cellular quality control machinery, the lack of an animal model system has limited studies to define the molecular mechanisms that result in the cellular pathology created by MT-COMP. Importantly and critical for this proposal, we have created a transgenic mouse model in which inducible chondrocyte-specific MT-COMP expression recapitulates the PSACH chondrocyte pathology. This MT-COMP mouse shows intracellular growth plate pathology in vivo and the chondrocytes can be isolated and utilized for in vitro studies. In these studies, we will use this novel and unique transgenic mouse to focus our efforts on: 1) defining the role of the Unfolded Protein Response (UPR) and activation of apoptosis in response to MT-COMP retention in chondrocytes in vivo, 2) testing the efficacy of different pharmacologic agents on inhibiting or reversing the intracellular retention of MT-COMP and 3) characterizing the effect of MT-COMP on articular cartilage morphology and function in mice throughout life. The MT-COMP mouse model allows us to test the hypothesis that removal of MT-COMP from the ER will return the chondrocyte to normal functionality. This is a first step towards a therapeutic intervention which will be a significant step forward as only symptomatic treatment is currently available for the painful osteoarthritis that is associated with PSACH and MED. Altogether, the results of this work will provide important information that will ultimately lead to better care of individuals with PSACH, MED/EDM1 and osteoarthritis.